Positional Touch screen sensors detect the location of an object (e.g. a finger or a stylus) applied to the surface of a touch screen display or the location of an object positioned near the surface of a touch screen display. These sensors detect the location of the object along the surface of the display, e.g. in the plane of a flat rectangular display. Examples of positional touch screen sensors include capacitive sensors, resistive sensors, and projected capacitive sensors. Such sensors include transparent conductive elements that overlay the display. The conductive elements can be combined with electronic components that can use electrical signals to probe the conductive elements in order to determine the location of an object near or in contact with the display.
In addition to positional sensing, it is often useful for user input devices (e.g., computer user input devices) such as touch screen displays to measure the magnitude of force associated with a touch event. A number of designs and constructions for touch screen display sensors that include force measurement have been described previously. Designs and constructions for touch screen display sensors that include force measurement include examples based on strain gauges such as is disclosed in U.S. Pat. No. 5,541,371 (Baller et al.); examples based on capacitance change between conductive traces or electrodes residing on different layers within the sensor, separated by a dielectric material or a dielectric structure comprising a material and air such as is disclosed in U.S. Pat. No. 7,148,882 (Kamrath et al.) and U.S. Pat. No. 7,538,760 (Hotelling et al.); examples based on resistance change between conductive traces residing on different layers within the sensor, separated by a piezoresistive composite material such as is disclosed in U.S. Pat. Publ. No. 2009/0237374 (Li et al.); and examples based on polarization development between conductive traces residing on different layers within the sensor, separated by a piezoelectric material such as is disclosed in U.S. Pat. Publ. No. 2009/0309616 (Klinghult et al.). Most such touch screen display sensors that measure the force of touch are limited by certain deficiencies. Those deficiencies include a propensity for transparent conductive elements (e.g., elements based on indium tin oxide) to crack when strained, conspicuous sensing elements that are unsuitable for interposing between a device user and an information display since they can substantially obscure the viewability of the display, and bulky force measurement components that can lead to undesirable device or bezel thickness or other large device dimensions.
In addition to conspicuous sensing elements that can substantially obscure the viewability of a display, greater thickness in the stack, particularly in the cover glass or film, may result in a reduced sensitivity of the touch sensor. Each of these problems are compounded or perpetuated in part because transparent conductive elements as well as supporting electronic structures, such as conductive metal interconnects, have been deposited or patterned onto films and/or glass substrates which ultimately become part of the final touch sensor optical stack.